One known spectrophotometer for measuring transmission spectra (cf. for example, Spectrophotometer Specord UV VIS described in Catalogue No. 32-3356-8, VEB Carl Zeiss Jena, pp. 4-6) comprises a light source, a single monochromator, a light detector connected to a recording means, and a holder with test sample placed between the monochromator and the light detector. This spectrophotometer allows measurement of sample transmission within the range of 100% to 3% of the incident light intensity, and for a range of wavelengths between 900 and 180 nm. Such a limited transmission measuring range is caused by a high diffuse light background in the monochromator.
Known in the art is a spectrophotometer for measuring transmission spectra (cf. for example, the sectrophotometer Acta M UV/VIS-NIR described in "Bulletin CR-7255", Beckman, Switzerland, 1975) differing from the device mentioned above in that a double monochromator is employed here rather than a single monochromator, for the purpose of diffuse light background suppression, the sequential arrangement of the component parts of the spectrophotometer remaining unaltered. This spectrophotometer allows transmission to be measured within the range of 100% to 0.1%, for the wavelengths between 900 and 180 nm. This transmission measuring range, however, is not satisfactory enough for present-day investigations.
Also known in the art is a spectrophotometer for measuring transmission spectra (cf. for example, a spectrophotometer described in the catalogue "Computerized UV/VIS and UV/VIS/NIR Absorption spectrophotometers" 320/330, Catalog N EX-E543 P, Hitachi Ltd, Tokyo, Japan) comprising a light source, a double monochromator, a light detector, and a recording means. Interposed between the output of the double monochromator and the light detector is a holder with a test sample secured thereto. The monochromator comprises a first slit, a first dispersive element, a second slit, a collimator mirror, and a second dispersive element all arranged in series along the propagation path of the light flux. The first slit disposed at the input of the double monochromator represents its input slit, and the second slit first guiding the light flux from the first dispersive element to the second dispersive element and then passing it out of the monochromator, serves a double purpose, namely it acts both as an intermediate slit of the monochromator and as its output slit, the direction of the light flux through the second slit being changed by the collimator mirror.
The use of high-precision components in this spectrophotometer allows extension of the transmission measuring range down to 0.1% compared to the aforementioned device, for the same wavelength range. For test samples having transmission values in excess of 0.01%, however, a sharp decrease in the measuring accuracy occurs. This is due to the fact that the luminescence background induced by the sample, as the incident light is partially absorbed, becomes comparable to intensity of the light that passed through the sample, and since the sample is positioned after the output slit of the monochromator and immediately before the light detector, this luminescence will be sensed by the light detector along with the light passed through the sample, causing the value of the signal detected by the recording means to be much in excess of that corresponding to the intensity of light passed through the sample. So luminescence of the sample presents difficulties in measuring small light fluxes passed through the sample, thus restricting the transmission measuring range to 0.01% and preventing measurement, with sufficient accuracy, of transmission spectra of samples made of luminescent and strong-absorbing materials, which limits the range of samples investigated.